The present invention relates generally to filters, and more particularly, to monolithic acoustically coupled thin-film piezoelectric filters.
Narrowband channel-select filters can bring significant power saving to RF communication systems. Large filter Q in excess of a few hundreds and small shape-factors are required to prevent cross-talk between closely-spaced channels.
Thin-film piezoelectric bulk acoustic resonators (FBAR) are utilized in the front-end of some transceiver circuits at GHz frequencies. See, for example, R. Ruby, et al., “Acoustic FBAR for filters, duplexers and front end modules,” IEEE MTT-S International Microwave Symposium Digest, Vol. 2, pp. 931-934, June 2004. Typically, a number of FBAR resonators are electrically connected in a ladder configuration to provide low-loss high-order filters with a very sharp roll-off skirt. To provide adequate out-of-band rejection, the number of resonators in the coupling chain of electrically-coupled filters should be considerably large (between 4 to 10). Given the relatively large size of each resonator, electrically coupled FBAR filters span over a large area, and may not offer integrated solutions for covering dispersed frequencies in a wide band. The size of the filter is of greater importance in emerging applications where multiple-band data transfer channels are required in a small form-factor.
Acoustic coupling of individual resonators can potentially offer much better out-of-band rejection in a small footprint. Electrical isolation between input and output ports of an acoustically-coupled filter is the key to reach large isolation in a low-order coupled system. Second-order stacked thin-film piezoelectric bulk acoustic wave (BAW) filters have been demonstrated with narrow pass-bands and excellent isolation suitable for applications where small size is critical. See, for example, Lakin, K. M., “Coupled resonator filters,” Proceesings of the IEEE 2002 Ultrasonics Symposium, vol. 1, pp. 901-908, 8-11 Oct., 2002.
The principle of operation of the presently-disclosed acoustically-coupled filters is similar to that of conventional stacked piezoelectric filters. However, the fabrication process in stacked filters is relatively complicated as multiple precisely-controlled thin-film deposition steps are involved, and thickness deviation can substantially degrade the performance of the filter. These filters also suffer from single-band operation on a chip. Monolithic quartz crystal filters are another example of acoustic filters widely utilized in IF frequency band. Exemplary quartz crystal filters are disclosed in U.S. Pat. Nos. 3,517,350 and 3,564,463 issued to Beaver, for example.
It would be desirable to have improved multi-center-frequency monolithic thin-film piezoelectric filters fabricated on a single chip which can operate at high frequencies.